1. Field of the Invention
The present invention relates to a liquid application device and an inkjet recording apparatus, and particularly to a liquid application device for applying liquid to a medium for a certain purpose which is, for example, to promote the coagulation of pigment when recording is carried out using an ink which contains the pigment as a coloring material. The present invention also relates particularly to an inkjet recording apparatus which includes a mechanism for applying liquid to a recording medium used in inkjet recording, for a purpose which is, for example, to promote the coagulation of pigment when recording is carried out using an ink containing the pigment as a coloring material.
2. Description of the Related Art
Conventionally, in the printing field, a configuration has been known in which an area where ink as application liquid is supplied to a roller is sealed off (refer to Japanese Patent Application Laid-open No. 8-058069 (1996)). An application mechanism described in this document is a mechanism of applying ink to the roller on whose surface, a printing plate pattern is formed in a gravure printing machine. Therein, the mechanism is configured to use an ink chamber having: doctor blades, which are located in two positions respectively corresponding to upper and lower locations along a circumferential surface of the roller, and which extend in a longitudinal direction of the roller; and elastic members respectively provided to both sides of these two doctor blades. By bringing this ink chamber into contact with the circumferential surface of the roller, a liquid room is formed between the chamber and the roller. Then, by rotating the roller, application liquid in this ink room is applied or supplied to the roller. In Japanese Patent Application Laid-open No. 08-058069 (1996) A, a pump is provided between an ink tank and the application liquid room, and operations such as supply of the application liquid are performed by using a pressurizing system. That is, the pump pumps ink inside the ink tank into the application liquid room under a pressure, and thereby the application liquid is supplied from the ink tank to the application liquid room, and the like.
Additionally, Japanese Patent Application Laid-open No. 2005-254229 proposes a liquid application device in which a sealing property in the above is further enhanced. In the liquid application device, an abutting portion of a liquid retention member is formed of a single member in an annular shape, and abuts on an application roller, thereby forming a liquid retention space for retaining application liquid. In Japanese Patent Application Laid-open No. 2005-254229, an elastic member is exemplified as the application roller abutting the liquid retention member.
In contrast to Japanese Patent Application Laid-open No. 8-058069 (1996), in the liquid application device in Japanese Patent Application Laid-open No. 2005-254229, a pump is provided at the side of a collection port of the liquid retention member, and the application liquid can be supplied, circulated and collected by using a negative pressure (decompression) system (pressure reduction system). In the case of the pressurizing system, liquid leakage may occur since a pressure inside a liquid room increases when a large amount of liquid is supplied to the application liquid room. By use of the negative pressure system, however, such liquid leakage can be reduced. Additionally, although the pressurizing system requires a supply control in which ink consumption by application is taken into consideration, the use of the negative pressure system eliminates the necessity of performing the supply control. Consequently, cost reduction and downsizing can be pursued. Thus, Japanese Patent Application Laid-open No. 2005-254229 has more advantages than Japanese Patent Application Laid-open No. 8-058069 (1996).
However, it is not mentioned in Japanese Patent Application Laid-open No. 2005-254229 that durability is uncertain when the elastic member is used as the application roller.
To be more precise, there has conventionally been a problem that, as the number of rotation times for applying liquid to recording media increases, the roller is more likely to become thinner by being shaved in a portion thereof where the recording media pass. That is, although the application liquid works also as a lubricant when adhering to the surface of the roller, the application liquid is applied to the recording medium by coming into contact with the recording medium. Subsequently, only a small amount of the application liquid is left on a region of the surface of the roller where the recording medium passes. At this time, if the region, where only the small amount of the application liquid is left, enters a nip area of the roller, a frictional force acting on the nip area becomes large.
In a region having no contact with the recording medium, a large amount of the application liquid remains. Consequently, if this region enters the nip area, the frictional force is small because the large amount of remaining application liquid works as a lubricant. On the other hand, in the region (the region having been in contact with the recording media) where only the small amount of application liquid is left, the frictional force becomes large as has been described above, and thereby abrasion of the roller may progress.
This problem is schematically shown in FIGS. 14 and 15. FIG. 15 conceptually shows how a roller 1401, which is shown in FIG. 14, having a predetermined roller diameter becomes after a durability test is given to the roller 1401, and also shows a graph regarding a relationship between the roller diameter and a position in the roller.
In FIGS. 14 and 15, right-hand ends of the drawings are references (a reference for transferring a recording medium) against which a recording medium abuts when the recording medium is transferred. When the recoding medium is transferred, the recording medium is transferred by abutting against the reference provided in a vicinity of an end portion of a device, or in a vicinity of the central portion thereof. Additionally, there are various sizes of recording media such as A4, A5, A3, B5 and B4 in the market. In order to handle these various sizes, a recording apparatus has a maximum width equal to that of recoding media which the recording apparatus supports. For example, in a case of an apparatus having, at a right-hand end thereof, a reference against which a recording medium abuts, frequencies at which recording media pass the roller are apparently different between a reference side of the roller and the other side (a non-reference side) thereof which is opposite to the reference side along a longitudinal direction of the application roller. This is because that recording media of any size pass the reference side of the roller, while some sizes of recording media do not pass the non-reference side thereof.
For this reason, amounts of abrasion of the roller become different between the reference side and the non-reference side. FIG. 14 illustrates an example of a roller of a recording apparatus in which a maximum size of supported recording media (a size of recording media on which the recording apparatus can perform recording) is A3. In a case where a user using this recording apparatus frequently uses the A4 size of recording media, as shown in FIG. 15, degrees at which the roller diameter decreases due to abrasion of the roller are more likely to differ between a part where A4 paper passes (a range 1501), and the other part (a range 1502). That is, a diameter of the part where A4 paper passes (the range 1501) may become thinner than that of the other part (the range 1502).
Additionally, after having filed Japanese Patent Application Laid-open No. 2005-254229, the inventors of the present invention discovered that the abrasion amount is more likely to be increased by a larger negative pressure inside the liquid retention member. FIG. 16 illustrates this discovery.
A pushing pressure of a cap against the roller changes when the diameter of the roller becomes thinner due to abrasion for these reasons. As explicitly mentioned in Japanese Patent Application Laid-open No. 2005-254229, the change of the pushing pressure leads to an unintentional change in the amount of the application. That is, uneven application occurs in a single recoding medium, whereby causing harmful effects on an image when the image is printed out.
Thus, although operations such as supply of application liquid by the negative pressure system have various advantages as has been described above, further contrivance for performing more favorable printing is needed.